Paper-based chemical assay devices include a paper substrate, wax that forms fluid channels and other fluid structures in the paper, and one or more reagents. Common examples of paper-based chemical assay devices include biomedical testing devices that are made of paper and perform biochemical assays and diagnostics in test fluids such as blood, urine and saliva. The devices are small, lightweight and low cost and have potential applications as diagnostic devices in healthcare, military and homeland security to mention a few.
Many of the paper-based diagnostic devices are formed from multiple layers of paper that is embedded with chemical reagents and with hydrophobic materials, such as wax or phase-change inks, that form channels to direct the diffusion of a biological fluid through the porous paper to one or more sites where the chemical reagents react with the biological fluid to perform the assay. When properly aligned, the multiple layers of paper enable three-dimensional paths for the fluid to reach the testing sites in the sensor, which enables a larger number of testing sites for different assays to be formed in a device of a given size compared to a two-dimensional arrangement in a single layer of paper. Additionally, some chemicals in the testing device are reactive to air or other environmental contaminants. The multiple layers of paper, with an optional coating of wax or another hydrophobic material, isolate portions of the testing device from the environment to prevent contamination.
Existing biomedical devices that are formed from multiple layers of paper or another substrate use adhesive layers that are interposed between the substrates to adhere multiple substrate layers together. For example, FIG. 6 depicts an exploded view of multiple paper layers 504A-504D that are bonded together with multiple corresponding layers of an adhesive film 508A-508C. Each layer of adhesive film is, for example, a two-sided adhesive tape. The adhesive film layers include holes, channels, and other perforations that enable liquid from one layer substrate layer to pass through the adhesive material to reach another layer of the substrate. For example, a fluid that is applied to the region 520 in the layer 504A passes through a corresponding opening 524 in the adhesive layer 508A to reach a corresponding fluid channel region 528 in the substrate layer 504B.
The separate adhesive layers that are used in the prior art biomedical have drawbacks during the manufacturing process and during use of biomedical sensors. During manufacture, the adhesive layers must be formed with openings that conform to the size, shape, and position of the openings in the two substrate layers that surround the adhesive. Forming the openings and aligning the substrate layers with the adhesive layer increases the complexity of the manufacturing process. During use, the biomedical sensor receives different biological fluids. In some instances, the biological fluids are chemically reactive with the adhesive material in the adhesive layers. The reactions between the fluid and the adhesive may contaminate the biomedical sensor and reduce the accuracy of assay results. Consequently, improvements to the production process and structure of multi-layer biomedical sensors and other multi-layer devices would be beneficial.